Various types of wireless systems are currently in use, including television broadcasting systems, cell-phone systems, and wireless local area networks (WLAN). Because frequency resources available across the entire society are limited, it is preferable to enable efficient use of the frequency resources in designing a wireless system. In this view, cognitive radio technology has been proposed that dynamically changes parameters, such as frequencies, according to the wireless environment in its vicinity so as to allow a number of wireless systems to coexist.
The cognitive radio technology provides the capability to share, amongst frequency resources allocated to wireless systems (primary users), frequency resources not in use by the primary users (sometimes referred to as “white spaces”) with other wireless systems (secondary users). The secondary users having no license are allowed to use a part of the frequency resources allocated to the primary users as long as they are able to sufficiently avoid interference with wireless devices of the primary users.
In the case of television broadcasting, for example, some physical channels are selected for each broadcast area from a plurality of physical channels allocated for television broadcasting. Therefore, in each broadcast area, a number of unused physical channels exist. In view of this, secondary usage of such unused physical channels (television white spaces) for wireless systems engaged in services other than television broadcasting has been discussed.
A white space database providing information on white spaces is sometimes installed to enable wireless systems identified as secondary users to detect white spaces. For example, a proposed television white space database provides a channel list indicating, amongst a plurality of television broadcasting physical channels, those currently not in use in a broadcast area. The usage of the white space database allows efficient search for a frequency resource sufficiently reducing interference between primary users and the secondary users.
A wireless communication system has been proposed in which a device of a primary user informs a device of a secondary user of frequency ranges allowed to use in the case where the device of the secondary user does not have a function of acquiring channel information indicating white spaces from a database. In addition, a coexistence system has been proposed that allows a secondary user network using white spaces and another wireless communication network using a communication system different from that of the secondary user network to coexist in the same frequency range, which is a white space frequency range.
See, for example, Japanese Laid-open Patent Publications No. 2012-5068 and No. 2012-60470.
In conventional technology, a wireless system operating as a secondary user continually (for example, periodically with a period of a few hours to a day) acquires frequency resource information from a database because frequency resources not in use by primary users may change. Then, when the conventional wireless system fails in reacquiring the frequency resource information at the time the frequency resource information is supposed to be reacquired, the wireless system stops its wireless communication until subsequently succeeding in the reacquisition.
Therefore, according to the conventional technology, when a failure occurs in the database or its peripheral network, disabling access to the database, the secondary usage of frequency resources is not available until the failure is corrected. That is, the database providing the frequency resource information acts as a bottleneck, which decreases the availability of the wireless system as a secondary user.